Plasma welding exhibits excellent heat concentration compared with TIG welding, and therefore welding can be performed at high speed with a narrow weld bead width, and strain can also be reduced.
One type of plasma welding method is keyhole welding, which is a one-side penetration welding method that uses a high energy density arc.
In keyhole welding, the plasma flow pushes aside the molten metal and penetrates through the base metal, forming a keyhole. As the welding progresses, the molten metal moves backward through the wall surface, and the keyhole forms a weld pool which becomes the weld bead.
As a result, the plate thickness that can be subjected to I-groove butt welding using one-pass one-side welding is generally not more than 6 mm for mild steel plate, and not more than approximately 8 mm for stainless steel plate.
Further, plasma welding uses tungsten for the electrode in the same manner as TIG welding, but because the tungsten electrode is located inside an insert chip, the electrode is not exposed to oxidizing gas, does not undergo sputtering, suffers minimal electrode wear, and can be used to perform high-quality welding for long periods, meaning running costs can be kept to a minimum.
Keyhole welding is widely used, mainly for manufacturing pressure vessels and piping joints, and for welding exhaust system components and the like. In this type of keyhole welding, for example, a dual shield nozzle torch (hereafter simply referred to as a “plasma welding torch”) such as that disclosed in Patent Document 1 is used.
The plasma welding torch includes a tungsten electrode, an insert chip which is disposed so as to surround the outer periphery of the tungsten electrode, and a shield cap which is disposed so as to surround the outer periphery of the insert chip.
The welding gas supplied to the plasma welding torch is composed of a pilot gas which is fed into the gap between the tungsten electrode and the insert chip, and a shield gas which is fed into the gap between the insert chip and the shield cap.
The pilot gas protects the tungsten electrode that generates the plasma, and also welds the welding workpiece. The shield gas prevents oxidation of the weld pool and the base material.
When keyhole welding is performed on an austenitic stainless steel, because the austenitic stainless steel contains Ni and the weld pool is viscous, welding can be performed without the weld pool itself falling.
On the other hand, when keyhole welding is performed on a ferritic stainless steel, the ferritic stainless steel does not contain Ni. Consequently, the weld pool of the ferritic stainless steel has a lower viscosity than the weld pool of an austenitic stainless steel, causing the weld pool itself to fall. As a result, keyhole welding cannot be performed.
Accordingly, the type of non-keyhole welding disclosed in Patent Document 2 is typically performed for welding ferritic stainless steel.
Further, Patent Document 2 discloses the use of argon gas or a mixed gas prepared by adding hydrogen to argon as the welding gas (the pilot gas and the shield gas) when performing non-keyhole welding.